Polymerisation process

ABSTRACT

Process for the polymerization of monomer in a polymerization system having at least one component attached thereto which is flushed with a flush medium which enters the polymerization system. Initially, the component is flushed with a first flush medium, and subsequently the component is flushed with a second flush medium which differs in composition from the first flush medium. A flush gas containing monomer is used as the flush medium when the polymer production rate is above 10 Te/h.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2012/076431 filed 20 Dec. 2012 which designated the U.S. andclaims priority to European Patent Application No. 11195903.7 filed 28Dec. 2011, the entire contents of each of which are hereby incorporatedby reference.

The present invention relates to a polymerisation process, and inparticular to the flushing of connections on a polymerisation reactionsystem.

BACKGROUND OF THE INVENTION

The polymerisation of monomer in both gas phase and slurry processes iswell-known and widely operated commercially. In either case monomer, andoptionally comonomer, are polymerised in the presence of apolymerisation catalyst to form the polymer.

The polymerisation reaction is highly exothermic. It is thereforeimportant to maintain efficient cooling throughout the reaction zone,and to prevent hot-spots. This is generally achieved by maintainingstrong circulation of reactants e.g. in a fluidised bed or slurry loop,and trying to avoid stagnant spots.

However, many of the connections to the reactor for feeding andwithdrawing materials, and for process monitoring or control, form zoneswhich have the potential for stagnant reaction medium. In particular,most connections comprise a valve and a line by which the valve isconnected to the reactor. When the valve is closed the line between thevalve and the reactor forms a zone with the potential for stagnantreaction medium. Reaction in these zones can form polymer, which mayfoul or block the line. Some of these connections may be linked tocritical services, such as compressor seals or relief valves, which losetheir effectiveness if exposed to the polymer/polymerisation reaction.

For this reason it is known to flush such zones both to prevent build-upof polymer in the zone and also to prevent contact of the polymerisationmixture with seals and the like.

This is usually done using a gas or liquid which is not harmful to thematerials, such as seals, in the zone, and also which is not overlydetrimental to the polymerisation reaction. Examples of suitablematerials in a polymerisation process can include inert gases such asnitrogen or inert liquids such as slurry reaction diluent.

Whilst nitrogen is a useful flush gas, in that it is inert and usuallyreadily available, an issue is that, although it does not directlyaffect the polymerisation, being inert it can build-up to relativelyhigh levels, and thus needs to be purged from the system. Being of asimilar molecular weight to ethylene, however, the purge often leads tolosses of significant quantities of valuable materials.

SUMMARY OF THE INVENTION

We have now found that an improved process comprises using differentflush mediums at different times.

Thus, the present invention provides a process for the polymerisation ofmonomer in a polymerisation system having at least one componentattached thereto which component is flushed with a flush medium whichenters the polymerisation system, wherein initially said component isflushed with a first flush medium, and subsequently said component isflushed with a second flush medium.

DETAILED DESCRIPTION OF THE INVENTION

“Polymerisation system” as used herein includes both a polymerisationreactor but also other parts of the overall apparatus in which flushpoints may be present. Thus, the term “polymerisation system” alsoincludes downstream processing steps, such as degassing steps andrecycle lines. The degassing steps may, for example, have compressorswith seals or filters which can be flushed.

A recycle line may, and generally will, include pumps which have sealswhich can be flushed.

The polymerisation system may be a slurry phase polymerisation system,for example for polymerisation of an olefin monomer in one or moreslurry loop reactors.

Preferably the polymerisation system is a gas phase polymerisationsystem, and most preferably a fluidised bed gas phase polymerisationsystem. The fluidised bed gas phase polymerisation system generallycomprises a fluidised bed gas phase reactor, a recycle system forcooling and recycling fluidising gas exiting the reactor back to thereactor, preferably after condensation of at least some of said stream,a polymer withdrawal line, downstream processing steps including atleast one degassing step for the withdrawn polymer, and at least onerecycle line for separated vapour from the downstream processing steps.

The at least one component may be any component attached to the reactionsystem that may be flushed. The term “component” as used herein refersto a component which is relatively small compared to the pipes or vesselto which it is connected and which make up the main parts of thereaction system. One example of components which may be flushed areseals, such as pump and compressor seals.

Other examples of components are pressure relief valves, withdrawallines, inlet lines, and instrument tappings. The “withdrawal fine” forexample, could be a polymer withdrawal line. The “inlet line” forexample, could be a catalyst injection line or a reactant feed line.“Stand-by” or “unused” withdrawal and inlet lines are generallymaintained with the manual or control valve for the line closed, and itis desired to flush the stagnant zone between the reactor and the valve.

Instrument tapping refers to a line by which an instrument is connectedto the reaction system. Examples include for flow measurement, fordensity measurement, for compositional analysis and, preferably, forpressure measurement (“pressure tapping”).

The term “flush”, as used herein, refers to the use of a flow of amedium (flush medium) to keep polymer and/or polymer reaction mediumaway from a component.

Thus, for example, the term “component” as used herein does notencompass vessels such as reactors and purge tanks, and the term“flushing” as used herein does not encompass purging of polymer toremove absorbed hydrocarbons.

Generally, there will be multiple components within the polymerisationsystem which may be flushed. Although some may in themselves have only arelatively small flush flow rate, the overall flow rate of flush mediuminto the reaction system can be significant. For example, where nitrogenis used as a flushing medium in all such components the total flow offlushing medium which may be required may be of the order of 400-800kg/h.

Preferably the present invention includes at least one component on thepolymerisation reactor itself. Components which are usually present onthe reactor itself and which are therefore preferred generally includepressure relief valves, withdrawal lines, inlet lines, and instrumenttappings.

The first and second flush mediums may be any mediums which differ incomposition. Usually one comprises an inert medium. However, and as longas the components are not a poison for the catalyst, the other maycomprise one or more non-inert components.

As used herein “non-inert components” refers to components which reactin the polymerisation system. In particular, we have found that reactivemediums can be used, and despite the reactive environment fouling of theflushed zones does not occur i.e. the flushing effect negates thepotential for additional reaction due to the use of a reactive medium.

This is surprising since it might have been expected that the higher theconcentration of a reactive medium at the site of potential fouling,especially monomer (or comonomer) concentration, then the higher therisk of polymerisation, and thereby fouling and/or blockage occurring.

Preferably the first and second flush mediums are first and second flushgases. In this case usually one comprises an inert gas or gases, and theother comprises one or more non-inert gases. In particular the one ormore non-inert gases may be one or more gases which are normally presentin the polymerisation system. Preferred examples of such gases are gasesthat are consumed in the reaction loop such as monomer (corresponding tothe monomer which is being polymerised), comonomers (when present andwhen gaseous under the reaction conditions) and hydrogen. Since they areconsumed accumulation of such components in the polymerisation system isnot an issue.

Where the first or second flush gas comprises an inert gas it preferablycomprises at least 90 wt % inert gas, especially at least 95 wt % inertgas, for example consists essentially of one or more inert gases. Thepreferred inert gas is nitrogen. Thus, the first flush gas preferablycomprises at least 90 wt % nitrogen, especially at least 95 wt %nitrogen, and most preferably the first flush gas consists essentiallyof nitrogen.

Where the first or second flush gas comprises monomer it may compriserelatively pure monomer, by which is meant at least 90 wt % monomer,especially at least 99 wt % monomer. In particular, a stream of freshmonomer may be used. However, it is also possible to use a processstream comprising lower levels of monomer. Such process streams, forexample as a recycle stream, are readily available, and may comprisemonomer, comonomer and nitrogen.

The switch from flushing with a first flush medium to flushing with asecond flush medium may be performed as a single step from one flushmedium to the other. Alternatively, the change may be made in more thanone step or even continuously by gradually replacing the first flushmedium with the second flush medium.

Preferably the switch is performed in a single step.

The first and second flush mediums may generally be introduced at anysuitable pressure, although they will generally be above the pressure atthe component where they are being used to ensure effective flushing.

The present invention may be advantageously applied during processstart-up, shut-down or a process upset.

In a most preferred embodiment the polymerisation system is a gas phasepolymerisation system and the first and second flush mediums are firstand second flush gases. The present invention will now be described withrespect to such a system although it will be apparent that a number ofthe advantages could equally be applied in other polymerisationprocesses and/or with liquid flush mediums. In a first embodiment, thefirst flush gas comprises an inert gas, preferably nitrogen.

The second flush gas in the first embodiment preferably comprises amonomer, and in particular the monomer being polymerised.

The first embodiment of the present invention may be advantageouslyapplied during process start-up, either from a planned or unplannedshut-down or upset.

In a start-up of a fluidised bed gas phase process, for example, a bedof polymer particles is usually fluidised prior to introduction ofcatalyst, and the reaction temperature increased. Prior to introductionof catalyst it is generally desired not to have too much monomerpresent. Once catalyst in injected then reaction starts at a controlledand relatively low level, and monomer feed rate can then be increased toincrease reaction rate.

Thus, a flush comprising inert gas can be used prior to catalystinjection so that monomer does not build-up to too high a level. Oncereaction has started, however, it has been found that a flush gascomprising monomer may be preferentially used. The advantage of a flushcomprising monomer is that monomer is desired in the process whenreaction is taking place. By minimising the introduction of nitrogen theamount that needs to be purged is minimised, and thereby so is theamount of monomer (and other valuable components) that is lost in thepurge.

A further advantage of a flush comprising monomer is that since themonomer is desired in the process when reaction is taking place it isalso possible to increase the flush rate without worrying about therequirement for an increased purge rate (as would be the case with aninert gas). Thus, the present invention allows to start-up with an inertflush, optionally at a relatively lower flush rate, and switch tomonomer, optionally at a relatively higher flush rate, as productionrate increases.

Another example of when the process of the first embodiment of thepresent invention may be applied is when re-starting after a processupset. In the event of an upset in which reaction must be rapidlystopped, for example a loss of cooling, it is usual to vent the reactorcontents. At this stage, to prevent further hydrocarbon emissions andalso to prevent a potentially flammable mixture building up again in thereaction system it has been found advantageous to switch the componentswhich need to be flushed to flushing with an inert gas. When the processis restarted then the flushes can be switched back to monomer accordingto the first embodiment of the present invention.

In a second embodiment the first flush gas may comprise the monomer andthe second flush may comprise an inert gas. As can be noted, this is theopposite of the first embodiment. An example of when such a process maybe applied is during the shut-down of a polymerisation process, orduring a process upset as described in the first embodiment.

As noted previously, the present invention may be advantageously appliedduring process start-up, shut-down or a process upset.

For example, preferably a flush gas comprising inert gas is used whenthe polymer production rate is below 5 Te/h

Preferably a flush gas comprising monomer is used when the polymerproduction rate is above 10 Te/h

As also noted above, generally, there will be multiple components withinthe polymerisation system which are flushed.

Although it is not necessary, the biggest benefits of the presentinvention are obtained when all of the components flushed with the firstflush medium are subsequently switched to be flushed with the secondflush medium. (It is not generally important whether they are switchedsimultaneously or sequentially however).

Where this is not done the biggest benefits obviously come fromswitching the components which have the highest flush rates. These aregenerally seals on pumps and compressors, which generally haverelatively high flush rates per se, and instrument tapping purges, whichwhilst individually have relatively low flush rates, collectively canrequire a significant flush rate.

The process for the polymerisation of monomer in a polymerisation systemmay otherwise be as known in the art. For example, the monomer to bepolymerised is not critical and the present invention can be applied topolymerisation of any suitable monomers. The monomer to be polymerisedis preferably an olefinic monomer, and most preferably is ethylene orpropylene. The monomer may be polymerised with a co-monomer, which maygenerally be an olefin other than the monomer, and having 2 to 8 carbonatoms. Thus, where ethylene is the monomer, suitable comonomers includepropylene, 1-butene, 1-hexene and 1-octene, and where propylene is themonomer, suitable comonomers include ethylene, 1-butene, 1-hexene and1-octene.

Equally, the catalyst used for the polymerisation is not critical, andany suitable polymerisation catalyst can be used. Examples ofpolymerisation catalysts include those based on chromium, Ziegler-Nattacatalysts and metallocene catalysts.

EXAMPLE

A fluidised bed polymerisation reaction is operated at a production rateof about 50 tonnes per hour to produce an ethylene/1-hexene co-polymer.The reaction system comprises a fluidised bed gas phase reactor, arecycle system for cooling, partially condensing the fluidising gasexiting the reactor and then recycling back to the reactor, whichincludes two compressors, a polymer withdrawal line with a downstreamdegassing step, and a recycle line for separated vapour from thedegassing step, also including a compressor.

The reactor itself has 16 flushed pressure tappings and a flushedcatalyst injection system.

In a comparative example, nitrogen is used for all flushes.Approximately 600 kg/h of nitrogen are introduced to the reaction systemvia the main loop recycle compressors, the degassing steps, thedegassing recycle compressor, the pressure tappings and the catalystinjection system.

This nitrogen needs to be purged from the system, and removes with itover 450 kg/hr of ethylene.

In an example according to the invention, nitrogen is used duringstart-up and until the production rate is 5 Te/h, at which point thepurges to the compressors and the pressure tappings are switched toethylene.

This reduced the nitrogen purge requirement by approximately 75%, andcorrespondingly the amount of nitrogen lost by the process.

Further, the removal of components such as ethylene and otherhydrocarbons with the nitrogen purge is reduced. This leads either to adirect reduction in the losses of such components (for example wherethey are flared) or to a reduction in the size of equipment to recoversuch components from nitrogen (for example, where ethylene is recoveredusing a membrane).

Another general advantage of a reduced purge rate is that ethane maybuild-up to a higher concentration in the reactor, high levels of ethanebeing generally advantageous.

The invention claimed is:
 1. A process for the polymerisation of monomerin a polymerisation system having at least one component attachedthereto which is flushed with a flush medium which enters thepolymerisation system, wherein initially said component is flushed witha first flush medium, and subsequently said component is flushed with asecond flush medium which second flush medium differs in compositionfrom the first flush medium, and wherein a flush gas comprising monomeris used as the flush medium when the polymer production rate is above 10Te/h.
 2. A process according to claim 1 wherein the at least onecomponent is selected from seals, pressure relief valves, withdrawallines, inlet lines, and instrument tappings.
 3. A process according toclaim 1 wherein there are multiple components attached to thepolymerisation system and which are flushed with a flush medium whichenters the polymerisation system.
 4. A process according to claim 1wherein the at least one component is attached on a polymerisationreactor and is selected from pressure relief valves, withdrawal lines,inlet lines, and instrument tappings.
 5. A process according to claim 1wherein one of the first and second flush mediums comprises an inertmedium, and the other comprises one or more non-inert components.
 6. Aprocess according to claim 1 wherein the polymerisation system is a gasphase polymerisation system.
 7. A process according to claim 1 whereinthe first and second flush mediums are first and second flush gases. 8.A process according to claim 7 wherein one of the first and second flushgases comprises an inert gas or gases and the other comprises one ormore non-inert gases.
 9. A process according to claim 8 wherein the oneor more non-inert gases include monomer, comonomers and hydrogen.
 10. Aprocess according to claim 7 wherein the first flush gas comprises aninert gas.
 11. A process according to claim 10 wherein the second flushgas comprises the monomer being polymerised.
 12. A process according toclaim 7 wherein the first flush gas comprises a monomer and the secondflush gas comprises an inert gas.
 13. A process according to claim 11wherein the second flush gas comprises relatively pure monomer, by whichis meant at least 90 wt % monomer.
 14. A process according to claim 11wherein the second flush gas is a process stream comprising monomer,comonomer and nitrogen.
 15. A process according to claim 12 wherein thefirst flush gas comprises relatively pure monomer, by which is meant atleast 90 wt % monomer.
 16. A process according to claim 12 wherein thefirst flush gas is a process stream comprising monomer, comonomer andnitrogen.
 17. A process according to claim 10 wherein the first flushgas comprises at least 90 wt % nitrogen.
 18. A process according toclaim 12 wherein the second flush gas comprises at least 90 wt %nitrogen.
 19. A process according to claim 6 wherein the polymerisationsystem is a fluidised bed gas phase polymerisation system.
 20. A processaccording to claim 10 wherein the first flush gas comprises nitrogen.21. A process according to claim 1 wherein the process starts-up with afirst flush medium comprising an inert medium and switches to the flushgas comprising monomer as production rate increases.
 22. A processaccording to claim 1 wherein a flush gas comprising inert gas is used asthe flush medium when the polymer production rate is below 5 Te/h.
 23. Aprocess according to claim 22 wherein the flush gas comprising an inertgas comprises at least 90 wt % nitrogen.